Method for transmitting and receiving frame performed by station operating in power save mode in wireless local area network system and apparatus for the same

ABSTRACT

A method for transmitting and receiving frames performed by a station (STA) in power save mode in a wireless local area network is provided. The method includes: transmitting a Power Save (PS) poll frame to an access point (AP) in a first 20 MHz channel; transmitting at least one duplicated PS poll frame to the AP in at least one second 20 MHz channel, the at least one duplicated PS poll frame being generated by duplicating the PS poll frame; and receiving a bufferable frame from the AP as a response of at least one of the PS poll frame and the at least one duplicated PS poll frame.

This application is the National Phase of PCT/KR2012/009058 filed onOct. 31, 2012, which claims priority under 35 U.S.C. 119(e) to USProvisional Application No. 61/594,378 filed on Feb. 3, 2012, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a wireless local area network system,and more particularly, to a method for transmitting and receiving framesperformed by a station operating in a power save mode in a wirelesslocal area network system and an apparatus for the same.

BACKGROUND ART

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate.

In a WLAN system, a station (STA) supports a power save mode. The STAenters a doze state to operate so that it is possible to prevent powerfrom being unnecessarily used. When traffic related to data to betransmitted to the STA that is operating in the doze state exists, anaccess point (AP) may indicate the traffic to the STA. The STA mayrecognize that the traffic related to the data to be transmitted theretoexists and may request the AP to transmit the traffic. The AP maytransmit a frame in response to the request of the STA.

Meanwhile, in a multi channel environment where data is exchangedthrough a plurality of channels, signaling for channels used for the APtransmitting the frame to the STA may be required. Therefore, anefficient method of transmitting and receiving a frame for the STA thatoperates in the power save mode in the multi channel environment isrequired.

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a method fortransmitting and receiving frames performed by a station (STA) operatingin a power save mode in a wireless local area network (WLAN) system andan apparatus for the same.

Solution to Problem

In an aspect, a method for transmitting and receiving frames performedby a station (STA) in power save mode in a wireless local area networkis provided. The method includes: transmitting a Power Save (PS) pollframe to an access point (AP) in a first 20 MHz channel; transmitting atleast one duplicated PS poll frame to the AP in at least one second 20MHz channel, the at least one duplicated PS poll frame being generatedby duplicating the PS poll frame; and receiving a bufferable frame fromthe AP as a response of at least one of the PS poll frame and the atleast one duplicated PS poll frame.

The PS poll frame and the at least one duplicate PS poll frame mayrespectively include a data field. The data field may be scrambled basedon a specific scrambling sequence. The data field may include atransmitter address (TA) field set to a bandwidth signaling TA. Thebandwidth signaling TA may indicate an address of the STA, and thespecific scrambling sequence may include first bandwidth information.

The first bandwidth information may indicate a bandwidth for the PS pollframe and the at least one duplicated PS poll frame.

The PS poll frame and the at least one duplicated PS poll frame mayfurther respectively include a VHT-SIG-A field including a bandwidthfield. The VHT-SIG-A field may be generated based on second bandwidthinformation. The second bandwidth information may be same as the firstbandwidth information.

The method may further include receiving an acknowledgement (ACK) framefrom the AP for acknowledging the PS poll frame and the at least oneduplicated PS poll frame.

The ACK frame may include a receiver address (RA) field. The RA fieldmay be set to a non-bandwidth signaling TA obtained in basis of the TAfield in at least one of the PS poll frame and the at least oneduplicated PS poll frame. The non-bandwidth signaling TA may indicatethe address of the STA, and include a individual/group bit set to ‘0’.

The number of the at least one secondary 20 MHz channel may be one, andthe first 20 Mhz channel may be contiguous with the secondary 20 MHzchannel.

The number of the at least one secondary 201 MHz channel may be three,and the first 20 MHz channel may be contiguous with the contiguous threesecondary 20 MHz channels.

The number of the at least one secondary 20 MHz channel may be seven,and the first 20 MHz channel may be contiguous with the contiguous sevensecondary 20 MHz channels.

The number of the at least one secondary 20 MHz may be seven, the first20 MHz channel may be contiguous with three 20 MHz secondary channelsamong the seven secondary 20 MHz channels, and the first 20 MHz channeland the three 20 MHz secondary channels may be not contiguous with theremaining four secondary 20 MHz channels.

The method may further include receiving a Traffic Indication Map (TIM)element from the AP; and determining whether the bufferable frame isbuffered for the STA based on the TIM element. If it is determined thatthe bufferable frame is buffered, the PS poll frame and the at least onePS poll frame may be transmitted.

In another aspect, a wireless apparatus operating in a wireless localarea network system is provided. The wireless apparatus includes atransceiver transmitting and receiving radio signals; and a processoroperably coupled to the transceiver. The processor is configured to:transmit a Power Save (PS) poll frame to an access point (AP) in a first20 MHz channel, transmit at least one duplicated PS poll frame to the APin at least one second 20 MHz channel, the at least one duplicated PSpoll frame being generated by duplicating the PS poll frame, and receivea bufferable frame from the AP as a response of at least one of the PSpoll frame and the at least one duplicated PS poll frame.

Advantageous Effects of Invention

In a multichannel wireless local area network (LAN) system, a station(STA) may request to transmit a buffered frame by transmitting a powersave (PS)-poll frame by a duplicate format and may signal a bandwidthfor transmitting the buffered frame. Therefore, an access point (AP) maytransmit the buffered frame to the STA based on information on thebandwidth signaled by the PS-poll frame. An STA that operates in a powersave mode in a multi channel environment may exchange a frame with an APthrough multi channel so that the throughput of the entire system may beimproved.

The STA that operates in the power save mode may transmit the SP-pollframe to the AP to request the AP to transmit the buffered frame and mayinitiate a polled service period for the above. The STA may receive atleast one buffered frame from the AP in the initiated polled serviceperiod. Since the STA may receive at least one frame by obtaining achannel access authority, the processing speed of the buffered trafficmay be improved. Therefore, the throughput of the entire system may beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

FIG. 2 shows a physical layer architecture of a WLAN system supported byIEEE 802.11.

FIGS. 3 and 4 are block diagrams illustrating the format of the PPDUused in the wireless LAN system according to an embodiment of thepresent invention.

FIG. 4 shows an example of a PPDU format used in the WLAN systemsupporting very high throughput (VHT).

FIG. 5 is a block diagram illustrating a format of an MAC frame providedby the WLAN system.

FIG. 6 is a block diagram illustrating a format of the HT control field.

FIG. 7 is a block diagram illustrating a format of an HT variant middlefield for HT.

FIG. 8 is a block diagram illustrating a format of an HT variant middlefield for VHT.

FIG. 9 shows an example of a power management operation.

FIG. 10 shows an example of a TIM element format.

FIG. 11 shows an example of a bitmap control field and a partial virtualbitmap field according to an embodiment of the present invention.

FIG. 12 is a flowchart illustrating an example of a responding procedureof an AP in a TIM protocol.

FIG. 13 is a flowchart illustrating another example of a responseprocedure of an AP in a TIM protocol.

FIG. 14 is a flowchart illustrating a procedure of a TIM protocol by aDTIM.

FIG. 15 shows an example of a method for transmitting and receivingframes by an STA that operates in a power save mode according to anembodiment of the present invention.

FIG. 16 shows another example of a method for transmitting and receivingframes by an STA operating in a power save mode according to anembodiment of the present invention.

FIG. 17 is a block diagram illustrating an MAC frame format of anSP-poll frame according to an embodiment of the present invention.

FIG. 18 shows an example of a method for transmitting and receivingframes by an STA that operates in a power save mode according to anotherembodiment of the present invention.

FIG. 19 shows another example of a method for transmitting and receivingframes by an STA that operates in a power save mode according to anotherembodiment of the present invention.

FIG. 20 shows still another embodiment of a method for transmitting andreceiving frames according to the embodiment of the present invention.

FIG. 21 shows a method for transmitting and receiving frames by an STAthat operates in a power save mode according to still another embodimentof the present invention.

FIG. 22 is a block diagram illustrating a wireless device by which anembodiment of the present invention may be realized.

MODE FOR THE INVENTION

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

Referring to FIG. 1, A WLAN system includes one or more Basic ServiceSet (BSSs). The BSS is a set of stations (STAs) which can communicatewith each other through successful synchronization. The BSS is not aconcept indicating a specific area

An infrastructure BSS includes one or more non-AP STAs STA1 21, STA2 22,STA3 23, STA4 24, and STAa 30, an AP (Access Point) 10 providingdistribution service, and a Distribution System (DS) connecting aplurality of APs. In the infrastructure BSS, an AP manages the non-APSTAs of the BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

An STA is a certain functional medium, including Medium Access Control(MAC) and wireless-medium physical layer interface satisfying theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standard. Hereinafter, the STA refers to both an AP and a non-AP STA.

A non-AP STA is an STA which is not an AP. The non-AP STA may also bereferred to as a mobile terminal, a wireless device, a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile subscriber unit, or simply a user. For convenience ofexplanation, the non-AP STA will be hereinafter referred to the STA.

The AP is a functional entity for providing connection to the DS througha wireless medium for an STA associated with the AP. Althoughcommunication between STAs in an infrastructure BSS including the AP isperformed via the AP in principle, the STAs can perform directcommunication when a direct link is set up. The AP may also be referredto as a central controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 canbe interconnected by the use of the DS. An extended service set (ESS) isa plurality of BSSs connected by the use of the DS. APs and/or STAsincluded in the ESS can communicate with each another. In the same ESS,an STA can move from one BSS to another BSS while performing seamlesscommunication.

In a WLAN system based on IEEE 802.11, a basic access mechanism of amedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordinate function (DCF) of the IEEE802.11 MAC, and basically employs a “listen before talk” accessmechanism. In this type of access mechanism, an AP and/or an STA sensesa wireless channel or medium before starting transmission. As a resultof sensing, if it is determined that the medium is in an idle status,frame transmission starts by using the medium. Otherwise, if it issensed that the medium is in an occupied status, the AP and/or the STAdoes not start its transmission but sets and waits for a delay durationfor medium access.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the AP and/or the STA directlysenses the medium. The virtual carrier sensing is designed to compensatefor a problem that can occur in medium access such as a hidden nodeproblem. For the virtual carrier sending, the MAC of the WLAN systemuses a network allocation vector (NAV). The NAV is a value transmittedby an AP and/or an STA, currently using the medium or having a right touse the medium, to anther AP or another STA to indicate a remaining timebefore the medium returns to an available state. Therefore, a value setto the NAV corresponds to a period reserved for the use of the medium byan AP and/or an STA transmitting a corresponding frame.

The IEEE 802.11 MAC protocol, together with a Distributed CoordinationFunction (DCF), provides a Hybrid Coordination Function (HCF) based on aPoint Coordination Function (PCF) of performing periodical polling byusing the DCF and a polling-based synchronous access method so that allreception APs or STAs or both can receive data packets. The HCF includescontention-based Enhanced Distributed Channel Access (EDCA) and HCFControlled Channel Access (HCCA) using a contention-free-based channelaccess scheme employing polling mechanism as access schemes used by aprovider in order to provide data packets to a plurality of users. TheHCF includes a medium access mechanism for improving Quality of Service(QoS) of a WLAN, and QoS data can be transmitted in both a ContentionPeriod (CP) and a Contention-Free Period (CFP).

In a wireless communication system, an STA cannot know the existence ofa network immediately when the STA is turned on and the STA startsoperating from a viewpoint of a wireless medium. Accordingly, any typeof an STA should perform a network discovery process in order to accessa network. The STA that has discovered networks through the networkdiscovery process selects a network to be joined through a networkselection process. Next, the STA joins the selected network and performsa data exchange operation performed in a transmission terminal/receptionterminal.

In a WLAN system, a network discovery process is embodied by a scanningprocedure. The scanning procedure is divided into passive scanning andactive scanning. Passive scanning is performed based on a beacon framethat is periodically broadcasted by an AP. In general, in a WLAN, an APbroadcasts a beacon frame at a specific interval (e.g., 100 msec). Thebeacon frame includes information about a BSS managed by the beaconframe. An STA waits passively in order to receive the beacon frame in aspecific channel. The STA obtains information about a network from thereceived beacon frame and then terminates the scanning procedure in thespecific channel. Passive scanning is advantageous in that overalloverhead is small because the passive scanning is performed if an STAhas only to receive a beacon frame without a need to transmit anadditional frame, but is disadvantageous in that the time taken toperform scanning is increased in proportion to the transmission periodof a beacon frame.

In contract, in active scanning, an STA broadcasts a probe request frameactively in a specific channel and requests information about networksfrom all APs that have received the probe request frame. An AP that hasreceived the probe request frame waists for a random time in order toprevent a collision between frames and transmits a probe response frame,including information about a network, to the STA. The STA receives theprobe response frame, obtains the information about networks from theprobe response frame, and then terminates the scanning procedure. Activescanning is advantageous in that scanning can be finished within arelatively short time, but is disadvantageous in that overall networkoverhead is increased because a frame sequence according to a requestand a response is necessary.

The STA that has finished the scanning procedure selects a networkaccording to its specific criterion and then, together with the AP,performs an authentication procedure. The authentication procedure isperformed according to a 2-way handshake. The STA that has finished theauthentication procedure, together with the AP, performs an associationprocedure.

The association procedure is performed according to a 2-way handshake.First, the STA transmits an association request frame to the AP. Theassociation request frame includes information about the capabilities ofthe STA. The AP determines whether or not to permit association with theSTA based on the information about the capabilities. The AP that hasdetermined whether or not to permit association with the STA transmitsan association response frame to the STA. The association response frameincludes information indicating whether association has been permittedor not and information indicating a reason when association is permittedor failed. The association response frame further includes informationabout capabilities supportable by the AP. If the association issuccessfully completed, frames are normally exchanged between the AP andthe STA. If the association is failed, the association procedure isattempted again based on information about a failure reason included inthe association response frame or the STA may request association fromanother AP.

In order to overcome a limit to the communication speed that wasconsidered as being weakness in a WLAN, IEEE 802.11n has been recentlyestablished as a technical standard. An object of IEEE 802.11n is toincrease the speed and reliability of a network and to extend thecoverage of a wireless network. More particularly, in order to support aHigh Throughput (HT) having a maximum data processing speed of 540 Mbpsor higher, minimize an error in transmission, and optimize the dataspeed, IEEE 802.11n is based on Multiple Inputs and Multiple Outputs(MIMO) technology using multiple antennas on both sides of a transmitterand a receiver.

As a WLAN is actively propagated and applications employing the WLAN arediversified, there is a need for a new WLAN system that supports athroughput higher than the data processing speed supported by IEEE802.11n. The next-generation WLAN system that supports a Very HighThroughput (VHT) is a next version of an IEEE 802.11n WLAN system and isone of IEEE 802.11 WLAN systems which have recently been newly proposedin order to support a data processing speed of 1 Gbps or higher in a MACService Access Point (SAP).

Further to a conventional WLAN system that supports 20 MHz and 40 MHz,in a VHT WLAN system, transmission of bandwidth of 80 MHz, contiguous160 MHz, and non-contiguous 160 MHz and/or transmission of bandwidth ofno less than 160 MHz is to be supported. Further to the conventionalwireless LAN system that supports up to 64 quadrature amplitudemodulation (QAM), the VHT wireless LAN system supports 256QAM.

Since a multiuser-multiple input multiple output (MU-MIMO) transmittingmethod is supported in the VHT wireless LAN system for higherthroughput, an AP may simultaneously transmit a data frame to at leastone MIMO paired STA. The maximum number of paired STAs may be 4. Whenthe maximum number of spatial streams is 8, up to 4 spatial streams maybe allotted to the STAs.

Referring back to FIG. 1, in a WLAN system, such as that shown in FIG.1, an AP 10 can transmit data to an STA group, including at least one ofa plurality of STAs 21, 22, 23, 24, and 30 associated therewith, at thesame time. An example where the AP performs MU-MIMO transmission to theSTAs is shown in FIG. 1. In a WLAN system supporting Tunneled DirectLink Setup (TDLS), Direct Link Setup (DLS), or a mesh network, however,an STA trying to send data may send a PPDU to a plurality of STAs byusing the MU-MIMO transmission scheme. An example where an AP sends aPPDU to a plurality of STAs according to the MU-MIMO transmission schemeis described below.

Data transmitted to the STAs may be transmitted through differentspatial streams. A data packet transmitted by the AP 10 as a physicallayer convergence procedure (PLCP) protocol data unit (PPDU) generatedby a physical layer of the wireless LAN system to be transmitted or adata field included in the PPDU may be referred to as a frame. That is,a PPDU for single user (SU)-MIMO and/or MU-MIMO or the data fieldincluded in the PPDU may be referred to as an MIMO packet. A PPDU for MUmay be referred to as an MU packet. In an example of the presentinvention, it is assumed that a group of STAs MU-MIMO paired with the AP10 to be transmitted includes an STA1 21, an STA2 22, an STA3 23, and anSTA4 24. At this time, spatial streams are not allotted to a specificSTA of the group of STAs to be transmitted so that data may not betransmitted. On the other hand, it is assumed that an STAa 30 iscombined with the AP, however, is not included in the group of STAs tobe transmitted.

In order to support MU-MIMO transmission in a WLAN system, an identifiermay be allocated to a target transmission STA group, and the identifiermay be called a group ID. An AP transmits a group ID management frame,including group definition information, to STAs supporting MU-MIMOtransmission in order to allocate a group ID to the STAs. The group IDis allocated to the STAs based on the group ID management frame prior toPPDU transmission. A plurality of group IDs may be allocated to one STA.

Table 1 below shows information elements included in the group IDmanagement frame.

TABLE 1 Order Information 1 Category 2 VHT action 3 Membership status 4Spatial stream position

The category field and the VHT action field are set to identify that theframe corresponds to a management frame and a group ID management frameused in the next-generation WLAN system supporting MU-MIMO.

As in Table 1, group definition information includes the membershipstatus information, indicating whether an STA belongs to a specificgroup ID, and spatial stream position information indicating that whatplace is the spatial stream set of a relevant STA located from all thespatial streams according to MU-MIMO transmission if the STA belongs tothe relevant group ID.

Since a plurality of group IDs is managed by one AP, membership statusinformation provided to one STA needs to indicate whether the STAbelongs to each of the group IDs managed by the AP. Accordingly, themembership status information may exist in an array form of subfields,indicating whether the STA belongs to each group ID. The spatial streamposition information may exist in an array form of subfields, indicatinga position of a spatial stream set occupied by an STA regarding eachgroup ID, because the spatial stream position information indicates aposition for each group ID. Furthermore, the membership statusinformation and the spatial stream position information for one group IDmay be implemented within one subfield.

If an AP transmits a PPDU to a plurality of STAs according to theMU-MIMO transmission scheme, the AP includes information, indicating agroup ID, in the PPDU, and transmits the information as controlinformation. When an STA receives the PPDU, the STA checks whether it isa member STA of a target transmission STA group by checking a group IDfield. If the STA is checked to be a member of the target transmissionSTA group, the STA may check that what place is a spatial stream set,transmitted thereto, placed from all the spatial streams. Since the PPDUincludes information about the number of spatial streams allocated to areception STA, the STA can receive data by searching for spatial streamsallocated thereto.

On the other hand, a TV white space (WS) is spotlighted as a frequencyband that may be newly used in the wireless LAN system. The TV WS refersto a frequency band in an idle state that is left due to digitalizationof analog TVs of the United States, for example, 54-698 MHz band.However, the above is only an example. The TV WS may be referred to as alicensed band that may be priorly used by a licensed user. The licenseduser means a user licensed to use the licensed band and may be referredto as a licensed device, a primary user, and an incumbent user.

The AP and/or the STA that operates in the TV WS must provide a functionof protecting the licensed user, which is because the licensed userpriorly uses the TV WS band. For example, when a specific WS channelthat is a frequency band divided to have specific bandwidth in the TV WSby regulation is previously used by the licensed user such as amicrophone, in order to protect the licensed user, the AP and/or the STAmay not use the frequency band corresponding to the corresponding WSchannel. In addition, when the frequency band that is currently used fortransmitting and/or receiving a frame is used by the licensed user, theAP and/or the STA must stop using the corresponding frequency band.

Therefore, a procedure of the AP and/or the STA determining whether thespecific frequency band in the TV WS band may be used, that is, whetherthe licensed user exists in the frequency band must be preceded.Determining whether the licensed user exists in the specific frequencyband is referred to as spectrum sensing. An energy detection method anda signature detection method are used as a spectrum sensing mechanism.It may be determined that the licensed user is using the frequency bandwhen the intensity of a received signal is no less than a predeterminedvalue or when a digital TV (DTV) preamble is detected.

FIG. 2 shows a physical layer architecture of a WLAN system supported byIEEE 802.11.

The IEEE 802.11 PHY architecture includes a PHY layer management entity(PLME), a physical layer convergence procedure (PLCP) sub-layer 210, anda physical medium dependent (PMD) sub-layer 200. The PLME provides a PHYmanagement function in cooperation with a MAC layer management entity(MLME). The PLCP sub-layer 210 located between a MAC sub-layer 220 andthe PMD sub-layer 200 delivers to the PMD sub-layer 200 a MAC protocoldata unit (MPDU) received from the MAC sub-layer 220 under theinstruction of the MAC layer, or delivers to the MAC sub-layer 220 aframe received from the PMD sub-layer 200. The PMD sub-layer 200 is alower layer of the PDCP sub-layer and serves to enable transmission andreception of a PHY entity between two STAs through a radio medium. TheMPDU delivered by the MAC sub-layer 220 is referred to as a physicalservice data unit (PSDU) in the PLCP sub-layer 210. Although the MPDU issimilar to the PSDU, when an aggregated MPDU (A-MPDU) in which aplurality of MPDUs are aggregated is delivered, individual MPDUs andPSDUs may be different from each other.

The PLCP sub-layer 210 attaches an additional field includinginformation required by a PHY transceiver in a process of receiving thePSDU from the MAC sub-layer 220 and delivering the PSDU to the PMDsub-layer 200. The additional field attached to the PSDU in this casemay be a PLCP preamble, a PLCP header, tail bits required to reset anconvolution encoder to a zero state, etc. The PLCP sublayer 210 receivesa TXVECTOR parameter, including control information necessary togenerate and transmit a Physical Layer Convergence Procedure (PLCP)Protocol Data Unit (PPDU) and control information necessary for areceiving STA to receive and interpret the PPDU, from the MAC sublayer220. The PLCP sublayer 210 uses the information included in the TXVECTORparameter in order to generate the PPDU including the PSDU.

The PLCP preamble serves to allow a receiver to prepare asynchronization function and antenna diversity before the PSDU istransmitted. In the PSDU, the data field may include padding bits, aservice field including a bit sequence for initializing a scrambler, anda coded sequence obtained by encoding a bit sequence to which tail bitsare attached. In this case, either binary convolutional coding (BCC)encoding or low density parity check (LDPC) encoding can be selected asan encoding scheme according to an encoding scheme supported in an STAthat receives a PLCP protocol data unit (PPDU). The PLCP header includesa field that contains information on a PPDU to be transmitted, whichwill be described below in greater detail with reference to FIGS. 3 to5.

The PLCP sub-layer 210 generates a PPDU by attaching the aforementionedfield to the PSDU and transmits the generated PPDU to a reception STAvia the PMD sub-layer. The reception STA receives the PPDU, acquiresinformation required for data recovery from the PLCP preamble and thePLCP header, and recovers the data. The PLCP sublayer of the receivingSTA transfers an RXVECTOR parameter, including control informationincluded in a PLCP preamble and a PLCP header, to an MAC sublayer sothat the MAC sublayer can interpret the PPDU and obtain data in areception state.

FIGS. 3 and 4 are block diagrams illustrating the format of the PPDUused in the wireless LAN system according to an embodiment of thepresent invention. Hereinafter, an STA that operates in a legacywireless LAN system based on IEEE 802.11a/b/g that is an existingwireless LAN standard prior to IEEE 802.11n is referred to as a legacySTA (L-STA). In addition, an STA that may support high throughput (HT)in an HT wireless LAN system based on the IEEE 802.11n is referred to asan HT-STA.

FIG. 3( a) illustrates the format of a legacy PPDU (L-PPDU) that is usedin the IEEE 802.11a/b/g that is the existing wireless LAN systemstandard prior to the IEEE 802.11n. Therefore, in the HT wireless LANsystem to which the IEEE 802.11n standard is applied, the L-STA maytransmit and receive the L-PPDU having the above format.

Referring to FIG. 3( a), an L-PPDU 310 includes an L-STF 411, an L-LTF312, an L-SIG field 313, and a data field 314.

The L-STF 311 is used for frame timing acquisition, automatic gaincontrol (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF 312 is used for frequency offset and channel estimation.

The L-SIG field 313 includes control information for demodulation anddecoding of the data field 314.

The L-PPDU may be transmitted in the order of the L-STF 311, the L-LTF312, the L-SIG field 313, and the data field 314.

FIG. 3( b) is a diagram showing an HT-mixed PPDU format in which anL-STA and an HT-STA can coexist. An HT-mixed PPDU 320 includes an L-STF321, an L-LTF 322, an L-SIG field 323, an HT-SIG field 324, an HT-STF325, a plurality of HT-LTF 326, and a data field 327.

The L-STF 321, the L-LTF 322, and the L-SIG field 323 are identical tothose shown in FIG. 3( a). Therefore, the L-STA can interpret the datafield by using the L-STF 321, the L-LTF 322, and the L-SIG field 323even if the HT-mixed PPDU 320 is received. The L-LTF 322 may furtherinclude information for channel estimation to be performed by the HT-STAin order to receive the HT-mixed PPDU 320 and to interpret the L-SIGfield 323, the HT-SIG field 324, and the HT-STF 325.

The HT-STA can know that the HT-mixed PPDU 320 is a PPDU dedicated tothe HT-STA by using the HT-SIG field 324 located next to the L-SIG field323, and thus can demodulate and decode the data field 327.

The HT-STF 325 may be used for frame timing synchronization, AGCconvergence, etc., for the HT-STA.

The HT-LTF 326 may be used for channel estimation for demodulation ofthe data field 327. Since the IEEE 802.11n supports single user-MIMO(SU-MIMO), a plurality of the HT-LTF 326 may be configured for channelestimation for each of data fields transmitted through a plurality ofspatial streams.

The HT-LTF 326 may consist of a data HT-LTF used for channel estimationfor a spatial stream and an extension HT-LTF additionally used for fullchannel sounding. Therefore, the number of the plurality of HT-LTF 326may be equal to or greater than the number of spatial streams to betransmitted.

The L-STF 321, the L-LTF 322, and the L-SIG field 323 are transmittedfirst so that the L-STA also can acquire data by receiving the HT-mixedPPDU 320. Thereafter, the HT-SIG field 324 is transmitted fordemodulation and decoding of data transmitted for the HT-STA.

Up to fields located before the HT-SIG field 324, transmission isperformed without beamforming so that the L-STA and the HT-STA canacquire data by receiving a corresponding PPDU. In the subsequentlyfields, i.e., the HT-STF 325, the HT-LTF 326, and the data field 327,radio signal transmission is performed by using precoding. In this case,the HT-STF 325 is transmitted so that an STA that receives a precodedsignal can consider a varying part caused by the precoding, andthereafter the plurality of HT-LTF 326 and the data field 327 aretransmitted.

Even if an HT-STA that uses 20 MHz in an HT WLAN system uses 52 datasubcarriers per OFDM symbol, an L-STA that also uses 20 MHz uses 48 datasubcarriers per OFDM symbol. Since the HT-SIG field 324 is decoded byusing the L-LTF 322 in a format of the HT-mixed PPDU 320 to supportbackward compatibility, the HT-SIG field 324 consists of 48×2 datasubcarriers. The HT-STF 325 and the HT-LTF 326 consist of 52 datasubcarriers per OFDM symbol. As a result, the HT-SIG field 324 issupported using ½ binary phase shift keying (BPSK), each HT-SIG field424 consists of 24 bits, and thus 48 bits are transmitted in total. Thatis, channel estimation for the L-SIG field 323 and the HT-SIG field 324is performed using the L-LTF 322, and a bit sequence constituting theL-LTF 322 can be expressed by Equation 1 below. The L-LTF 322 consistsof 48 data subcarriers per one symbol, except for a DC subcarrier.L_(−26,26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}  [Math.1]

FIG. 3( c) is a diagram showing a format of an HT-Greenfield PPDU 330that can be used by only an HT-STA. The HT-GF PPDU 330 includes anHT-GF-STF 331, an HT-LTF1 332, an HT-SIG field 333, a plurality ofHT-LTF2 334, and a data field 335.

The HT-GF-STF 331 is used for frame timing acquisition and AGC.

The HT-LTF1 332 is used for channel estimation.

The HT-SIG field 333 is used for demodulation and decoding of the datafield 335.

The HT-LTF2 334 is used for channel estimation for demodulation of thedata field 335. Since the HT-STA uses SU-MIMO, channel estimation isrequired for each of data fields transmitted through a plurality ofspatial streams, and thus a plurality of HT-LTF2 334 may be configured.

The plurality of HT-LTF2 334 may consist of a plurality of data HT-LTFsand a plurality of extension HT-LTFs, similarly to the HT-LTF 326 of theHT-mixed PPDU 320.

Data fields 314, 327, and 335 illustrated in FIG. (a), (b), and (c) mayinclude a service field, a scrambled PLCP service data unit (PSDU), atail bit, and a padding bit, respectively. The service field may be usedfor initializing a scrambler. The service field may be configured by 16bits. In this case, bits for initializing the scrambler may be realizedby 7 bits. The tail field may be configured by a bit sequence requiredfor returning a convolution encoder to a zero state. A bit sizeproportional to the number of binary convolutional code (BCC) encodersused for encoding data to be transmitted may be allotted to the tailfield. In detail, the tail field may be realized to have 6 bits by thenumber of BCCs.

FIG. 4 shows an example of a PPDU format used in the WLAN systemsupporting very high throughput (VHT).

Referring to FIG. 4, a PPDU 400 includes an L-STF 410, an L-LTF 420, anL-SIG field 430, a VHT-SIGA field 440, a VHT-STF 450, a VHT-LTF 460, aVHT-SIGB field 470, and a data field 480.

A PLCP sub-layer constituting a PHY converts a PSDU delivered from a MAClayer into the data field 480 by appending necessary information to thePSDU, generates the PPDU 400 by appending several fields such as theL-STF 410, the L-LTF 420, the L-SIG field 430, the VHT-SIGA field 440,the VHT-STF 450, the VHT-LTF 460, the VHT-SIGB field 470, or the like,to the data field and delivers the PPDU 400 to one or more STAs througha physical medium dependent (PMD) sub-layer constituting the PHY.Control information required by the PLCP sub-layer to generate the PPDUand control information used by a reception STA to interpret the PPDUand transmitted by being included in the PPDU are provided from aTXVECTOR parameter delivered from the MAC layer.

The L-SFT 410 is used for frame timing acquisition, automatic gaincontrol (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF 420 is used for channel estimation for demodulation of theL-SIG field 430 and the VHT-SIGA field 440.

The L-SIG field 430 is used when the L-STA receives the PPDU 400 andinterprets it to acquire data. The L-SIG field 430 includes a ratesub-field, a length sub-field, a parity bit and tail field. The ratesub-field is set to a value indicating a bit state for data to becurrently transmitted.

The length sub-field is set to a value indicating an octet length of aPSDU to be transmitted by the PHY layer at the request of the MAC layer.In this case, an L_LENGTH parameter which is a parameter related toinformation indicating the octet length of the PSDU is determined basedon a TXTIME parameter which is a parameter related to a transmissiontime. TXTIME indicates a transmission time determined for PPDUtransmission including the PSDU by the PHY layer in association with atransmission time requested for transmission of the PSDU. Therefore,since the L_LENGTH parameter is a time-related parameter, the lengthsub-field included in the L-SIG field 430 includes information relatedto the transmission time.

The VHT-SIGA field 440 includes control information (or signalinformation) required by STAs for receiving the PPDU to interpret thePPDU 400. The VHT-SIGA 440 is transmitted on two OFDM symbols.Accordingly, the VHT-SIGA field 440 can be divided into a VHT-SIGA1field and a VHT-SIGA2 field. The VHT-SIGA1 field includes channelbandwidth information used for PPDU transmission, identifier informationrelated to whether space time block coding (STBC) is used, informationindicating either SU or MU-MIMO as a PPDU transmission scheme, and, ifthe transmission scheme is MU-MIMO, information indicating atransmission target STA group of a plurality of STAs which are MU-MIMOpaired with the AP, and information regarding a spatial stream allocatedto each STA included in the transmission target STA group. The VHT-SIGA2field includes information related to a short guard interval (GI).

The information indicating the MIMO transmission scheme and theinformation indicating the transmission target STA group can beimplemented as one piece of MIMO indication information, and forexample, can be implemented as a group ID. The group ID can be set to avalue having a specific range. A specific value in the range indicatesan SU-MIMO transmission scheme, and other values can be used as anidentifier for a corresponding transmission target STA group when theMU-MIMO transmission scheme is used to transmit the PPDU 400.

When the group ID indicates that the PPDU 400 is transmitted using theSU-MIMO transmission scheme, the VHT-SIGA2 field includes codingindication information indicating whether a coding scheme applied to thedata field is binary convolution coding (BCC) or low density paritycheck (LDPC) coding and modulation coding scheme (MCS) informationregarding a channel between a transmitter and a receiver. In addition,the VHT-SIGA2 field can include an AID of a transmission target STA ofthe PPDU and/or a partial AID including a part of bit-sequence of theAID.

When the group ID indicates that the PPDU 400 is transmitted using theMU-MIMO transmission scheme, the VHT-SIGA field 400 includes codingindication information indicating whether a coding scheme applied to thedata field which is intended to be transmitted to MU-MIMO pairedreception STAs is BCC or LDPC coding. In this case, MCS information foreach reception STA can be included in the VHT-SIGB field 470.

The VHT-STF 450 is used to improve performance of AGC estimation in MIMOtransmission.

The VHT-LTF 460 is used when the STA estimates a MIMO channel. Since thenext generation WLAN system supports MU-MIMO, the VHT-LTF 460 can beconfigured by the number of spatial streams in which the PPDU 400 istransmitted. In addition, when full channel sounding is supported and isperformed, the number of VHT-LTFs may increase.

The VHT-SIGB field 470 includes dedicated control information requiredwhen the plurality of MIMO-paired STAs receive the PPDU 400 to acquiredata. Therefore, the STA may be designed such that the VHT-SIGB field470 is decoded only when the control information included in theVHT-SIGA field 440 indicates that the currently received PPDU 400 istransmitted using MU-MIMO transmission. On the contrary, the STA may bedesigned such that the VHT-SIGB field 470 is not decoded when thecontrol information in the VHT-SIGA field 440 indicates that thecurrently received PPDU 400 is for a single STA (including SU-MIMO).

The VHT-SIGB field 470 may include MCS information and rate-matchinginformation for each STA. Further, the VHT-SIGB field 470 may includeinformation indicating a PSDU length included in the data field for eachSTA. The information indicating the PSDU length is informationindicating a length of a bit-sequence of the PSDU and can be indicatedin the unit of octet. Meanwhile, when the PPDU is transmitted based onsingle user transmission, the information about the MCS may not beincluded in the VHT-SIGB field 470, because that is included in theVHT-SIGA field 440. A size of the VHT-SIGB field 470 may differaccording to the MIMO transmission method (MU-MIMO or SU-MIMO) and achannel bandwidth used for PPDU transmission.

The data field 480 includes data intended to be transmitted to the STA.The data field 480 includes a PLCP service data unit (PSDU) to which aMAC protocol data unit (MPDU) of a MAC layer is delivered, a servicefield for initializing a scrambler, a tail field including a bitsequence required to reset a convolution encoder to a zero state, andpadding bits for normalizing a length of the data field. In case of MUtransmission, each data unit intended to be respectively transmitted toeach STA may be included in the data field 580. The data unit may beaggregate MPDU (A-MPDU).

In the WLAN system of FIG. 1, if the AP 10 intends to transmit data tothe STA1 21, the STA2 22, and the STA3 23, then a PPDU may betransmitted to an STA group including the STA1 21, the STA2 22, the STA323, and the STA4 24. In this case, as shown in FIG. 4, no spatial streammay be allocated to the STA4 24, and a specific number of spatialstreams may be allocated to each of the STA1 21, the STA2 22, and theSTA3 23 and thus data can be transmitted. In the example of FIG. 4, onespatial stream is allocated to the STA1 21, three spatial streams areallocated to the STA2 22, and two spatial streams are allocated to theSTA3 23.

FIG. 5 is a block diagram illustrating a format of an MAC frame providedby the WLAN system. The MAC frame may be an MAC protocol data unit(MPDU) (a PSDU when transmitted to a PHY layer) included in the datafield of the above-described PPDU.

Referring to FIG. 5, an MAC frame 500 includes a frame control field510, a duration/ID field 520, an address 1 field 531, an address 2 field532, an address 3 field 533, a sequence control field 540, an address 4field 534, a quality of service (QoS) control field 550, an HT controlfield 560, a frame body 570, and a frame check sequence (FCS) field 580.

The frame control field 510 includes information on the characteristicsof a frame. The frame control field may include protocol versioninformation that indicates the version of the WLAN standard supported bythe frame 500 and information on a type and subtype for identifying thefunction of the frame.

The duration/ID field 520 may be realized to have different values inaccordance with the type and subtype of the frame 500. When the frame500 is determined as the PS-poll frame for power save operation by thetype and subtype of the frame 500, the duration/ID field 520 may beconfigured to include the association identifier (AID) of the STA thattransmits the frame 500. In the other cases, the duration/ID field 520may be configured to have a specific duration value in accordance withthe type and subtype of the frame 500. When the frame 500 is the MPDUincluded in an A-MPDU format, the duration/ID fields 520 included in theMAC headers of the MPDUs may be realized to have the same value.

The address 1 field to the address 4 field 531 to 534 may be configuredto realize specific fields among a basic set service identification(BSSID) field for indicating BSSID, a source address (SA) field forindicating an SA, a destination address (DA) field for indicating a DA,a transmitting address (TA) field for indicating a transmitted STAaddress, and a receiving address (RA) field for indicating a receivedSTA address. Meanwhile, an address field realized as a TA field may beconfigured to indicate a bandwidth signaling TA value. In this case, theTA field may indicate that the frame contains additional information ina scrambling sequence. The bandwidth signaling TA may be represented asa MAC address of a STA transmitting the relevant frame, and anindividual/group bit in the MAC address may be set to a predeterminedvalue, e.g. ‘1’.

The sequence control field 540 is configured to include a sequencenumber and a fragment number. The sequence number may indicate thesequence number allotted to the frame 500. The fragment number mayindicate the numbers of the fragments of the frame 500.

The QoS control field 550 includes information on QoS.

The HT control field 560 includes control information on an HTtransmitting and receiving method and/or a VHT transmitting andreceiving method. Realization of the HT control field 560 will bedescribed in detail hereinafter.

The frame body 570 may include data to be transmitted by a transmittedSTA and/or AP. Body components excluding an MAC header and an FCS from acontrol frame, a management frame, an action frame, and/or a data frameto be transmitted may be realized in the frame body 570. When the frame500 is the management frame and/or the action frame, informationelements included in the management frame and/or the action frame may berealized in the frame body 570.

The FCS field 580 includes a bit sequence for cyclic redundancy check(CRC).

Hereinafter, the above-described HT control field will be described indetail with reference to the drawing.

FIG. 6 is a block diagram illustrating a format of the HT control field.

Referring to FIG. 6, the HT control field 560 includes a VHT variantfield 561, an HT control middle field 562, an access category (AC)constraint field 563, and an RDG/More PPDU field 564.

The VHT variant field 561 indicates whether the HT control field 560 hasa format of an HT control field for VHT or a format of an HT controlfield for HT. For example, the VHT variant field 561 may be realized bya field having length of 1 bit. According to the value, it may beindicated whether the HT control middle field 562 is realized by theformat for HT or the format for VHT.

The HT control middle field 562 may be realized to have another formatsin accordance with the indication of the VHT variant field 561. Detailedrealization of the HT control middle field 562 will be described indetail hereinafter.

The AC constraint field 563 indicates whether a mapped AC of a reversedirection (RD) data frame is limited to a single AC.

The RDG/More PPDU field 564 may be differently interpreted in accordancewith the corresponding filed is transmitted by an RD initiator or an RDresponder. In the case where the corresponding field is transmitted bythe RD initiator, when the RDG/More PPDU field is configured as ‘1’, itmay be interpreted that the RDG exists and may be defined by theduration/ID field. In the case where the corresponding field istransmitted by the RD responder, when the RDG/More PPDU field isconfigured as ‘0’, it may be interpreted as indicating that the PPDUincluding the field is the final frame transmitted by the RD responder.When the RDG/More PPDU field is configured as ‘1’, it may be interpretedas indicating another PPDU is transmitted following the PPDU includingthe field.

FIG. 7 is a block diagram illustrating a format of an HT variant middlefield for HT.

Referring to FIG. 7, an HT variant middle field 700 for HT includes alink adaptation control subfield 710, a calibration position sub field720, a calibration sequence subfield 730, a channel state information(CSI)/steering subfield 740, a null data packet (NDP) announcementsubfield 750.

The link adaptation control subfield 710 includes a training request(TRQ) subfield 711, an modulation coding scheme (MCS) request (MRQ) orantenna selection (ASEL) indication (MAI) subfield 712, an MCAS feedbacksequence identifier (MFSI) subfield 713, and an MCS feedback and antennaselection command/data (MFB/ASELC) subfield 714.

The TRQ subfield 711 includes information requesting a soundingresponder to transmit a sounding frame. The MAI subfield 712 may includeindication information requesting an MCS feedback or informationindicating that the MFB/ASELC subfield 714 includes ASEL information.The MAI subfield 712 may include an MRQ sequence identifier (MSI)subfield including an MCS request (MRQ) indication bit and a sequencenumber for identifying MRQ. Whether the MCS feedback is requested may beindicated by configuring a value of a subfield. The MFSI subfield 713may be configured by a received value of the MSI included in the framerelated to the MFB information. The MFB/ASELC subfield 714 includes theMFB information or the ASEL information.

The calibration position subfield 720 and the calibration sequencesubfield 730 include the position of a calibration sounding exchangesequence and identification information on a calibration sequence.

The CSI/steering subfield 740 indicates information indicating afeedback type.

The NDP announcement subfield 750 may be configured as NDP announcementindication information that announces that an NDP is to be transmittedfollowing the currently transmitted PPDU. The NDP announcement subfield750 may be configured by a field of 1 bit. An STA that receives a PPDUmay confirm whether the corresponding PPDU is an NDPA frame through thevalue of the NDP announcement subfield 750.

FIG. 8 is a block diagram illustrating a format of an HT variant middlefield for VHT.

Referring to FIG. 8, an HT variant middle field 800 for VHT includes anMRQ subfield 810, an MSI subfield 820, an MFSI/GID-L subfield 830, anMFB subfield 840, a GID-H subfield 850, a coding type subfield 860, anFB Tx type subfield 870, and a unsolicited MFB subfield 880.

The MRQ subfield 810 indicates whether the MCS feedback is requested.The MRQ subfield 810 may be realized as requesting the MCS feedbackconfigured as ‘1’.

When the MRQ subfield 810 indicates that the MCS feedback is requested,the MSI subfield 820 includes a sequence number identifying the specificrequest.

The unsolicited MFB subfield 880 may indicate whether the included MFBinformation is a response to the MRQ. When the unsolicited MFB subfield880 is configured as ‘1’, the included MFB information may be realizedas a response to the MRQ. When the unsolicited MFB subfield 880 isconfigured as ‘0’, the included MFB information may be realized as notbeing a response to the MRQ.

The MFSI/GID-L subfield 830 may be differently interpreted in accordancewith the configuration of the unsolicited MFB subfield 880. When theunsolicited MFB subfield 880 indicates that the included MFB informationis a response to the MRQ, the received value of the MSI included in theframe related to the MFB information may be included. When theunsolicited MFB subfield 880 indicates that the included MFB informationis not a response to the MRQ, the lowest 3 bits that configure the IDsof a group of PPDUs related to the unsolicited MFB information may beincluded.

The MFB subfield 840 may include recommended MFB information. The MFBsubfield 840 may include a VHT N_STS subfield 841, an MCS subfield 842,a BW subfield 843, and a signal-to-noise (SNR) subfield 844. The VHTN_STS subfield 841 indicates the number of recommended spatial streams.The MCS subfield 842 indicates a recommended MCS. The BW subfield 843indicates bandwidth information related to the recommended MCS. The SNRsubfield indicates an average SNR value on data subcarriers and spatialstreams.

When the unsolicited MFB subfield 880 indicates that the MFB informationis not a response to the MRQ and the MFB is estimated from a PPDU fortransmitting and receiving an MU, the GID-H subfield 850 may include thehighest 3 bits that configure the IDs of the group of the PPDUs relatedto the unsolicited MFB information. When the MFB is estimated from aPPDU for transmitting receiving an SU, the GID-H subfield 850 mayinclude a bit sequence configured as 1.

When the unsolicited MFB subfield 880 indicates that the MFB informationis not a response to the MRQ, the coding type subfield 860 may includecoding information (BCC or LDPC) on the frame in which the unsolicitedMFB information is estimated.

The FB Tx type subfield 870 may be configured to indicate thetransmission type of an estimated PPDU. That is, the FB Tx type subfield870 may indicate whether the estimated PPDU is beamformed.

The VHT variant field 561 may distinguish the HT control field for VHTfrom the HT control field for HT based on the control informationincluded in the HT control middle field 562.

On the other hand, a next generation wireless LAN system supportstransmission of an MU-MIMO method in which a plurality of STAssimultaneously access a channel in order to efficiently use a wirelesschannel. According to the MU-MIMO transmitting method, an AP maysimultaneously transmit a packet to at least one MIMO paired STA.

Meanwhile, if channel sensing is always performed for frame transmissionand reception, it causes persistent power consumption of the STA. Sincepower consumption in a reception state is not much different from powerconsumption in a transmission state, if the reception state needs to becontinuously maintained, relatively great power consumption is generatedin an STA that operates by using a battery. Therefore, when the STAsenses a channel by persistently maintaining a reception standby statein a WLAN system, ineffective power consumption may be caused without aspecial synergy effect in terms of a WLAN throughput, and thus it may beinappropriate in terms of power management.

To compensate for the problem above, the WLAN system supports a powermanagement (PM) mode of the STA. A power management (PM) mode of a STAis classified into an active mode and a power save (PS) mode in a WLANsystem. Basically, the STA operates in the active mode. When operatingin the active mode, the STA can operate in an awake state so that aframe can be received all the time.

When operating in the PS mode, the STA operates by transitioning betweena doze state and the awake state. When operating in the doze state, theSTA operates with minimum power, and does not receive a radio signal,including a data frame, transmitted from an AP. In addition, the STAoperating in the doze state does not perform channel sensing.

The longer the STA operates in a doze state, the less the powerconsumption is, and thus the longer the STA operates. However, since aframe cannot be transmitted and received in the doze state, the STAcannot operate long unconditionally. If the STA operating in the dozestate has a frame to be transmitted to the AP, the STA can transition toan awake state to transmit the frame. However, if the AP has a frame tobe transmitted to the STA operating in the doze state, the STA cannotreceive the frame and cannot know that there is the frame to bereceived. Therefore, the STA may need to know whether there is the frameto be transmitted to the STA, and if the frame exists, may require anoperation for transitioning to the awake state in accordance with aspecific period. According to this operation, the AP can transmit theframe to the STA. This will be described with reference to FIG. 9.

FIG. 9 shows an example of a power management operation.

Referring to FIG. 9, an AP 910 transmits a beacon frame to STAs in a BSSin accordance with a specific period (step S910). The beacon frameincludes a traffic indication map (TIM) information element. The TIMelement includes information for indicating that the AP 910 has abufferable frame (or bufferable unit) for the STAs associated with theAP being buffered and the frame will be transmitted. Examples of the TIMelement include a TIM used to report a unicast frame and a deliverytraffic indication map (DTIM) used to report a multicast or broadcastframe.

The AP 910 transmits the DTIM one time whenever a beacon frame istransmitted three times.

An STA1 921 and an STA2 922 are STAs operating in a PS mode. The STA1921 and the STA2 922 can be configured such that they can transitionfrom a doze state to an awake state in every wakeup interval of aspecific period to receive the TIM element transmitted by the AP 910.

A specific wakeup interval can be configured such that the STA1 921transitions to the awake state in every beacon interval to receive theTIM element. Therefore, the STA1 921 transitions to the awake state(step S921) when the AP 910 transmits a first beacon frame (step S911).The STA1 921 receives the beacon frame and acquires the TIM element. Ifthe acquired TIM element indicates that a bufferable frame to betransmitted to the STA1 921 is buffered, then the STA1 921 transmits tothe AP 910 a PS poll frame that requests the AP 910 to transmit a frame(step S921 a). The AP 910 transmits the frame to the STA1 921 inresponse to the PS poll frame (step S931). Upon completion of framereception, the STA1 921 operates by transitioning back to the dozestate.

When the AP 910 transmits a second beacon frame, a medium is busy, thatis, another device accesses to the medium for example. Thus, the AP 910may not be able to transmit the beacon frame in accordance with acorrect beacon interval but may transmit it at a delayed time point(step S912). In this case, the STA1 921 switches its mode to the wakestate in accordance with the beacon interval, but cannot receive thebeacon frame transmitted with delay, and thus transitions back to thedoze state (step S922).

When the AP 910 transmits a third beacon frame, the beacon frame mayinclude a TIM element which is configured as a DTIM. However, since themedium is busy, the AP 910 transmits the beacon frame with delay (stepS913). The STA1 921 operates by transitioning to the awake state inaccordance with the beacon interval, and can acquire the DTIM by usingthe beacon frame transmitted by the AP 910. The DTIM acquired by theSTA1 921 indicates that there is no frame to be transmitted to the STA1921 and there is a frame for another STA. Therefore, the STA1 921operates by transitioning back to the doze state. After transmitting thebeacon frame, the AP 910 transmits the frame to a corresponding STA(step S932).

The AP 910 transmits a fourth beacon frame (step S914). However, sincethe STA1 921 cannot acquire information indicating that there isbuffered traffic for the STA1 921 by receiving the TIM element twotimes, the STA1 921 may regulate a wakeup interval for receiving the TIMelement. Alternatively, if signaling information for regulating a wakeupinterval value of the STA1 921 is included in the beacon frametransmitted by the AP 910, the wakeup interval value of the STA1 921 maybe regulated. Instead of transitioning an operation state for everybeacon interval to receive the TIM element, the STA1 921 can beconfigured in the present embodiment such that the operation state istransitioned one time for every three beacon intervals. Therefore, theSTA1 921 cannot acquire a corresponding TIM element since the AP 910transmits the fourth beacon frame (step S914), and maintains the dozestate when a fifth beacon frame is transmitted (step S915).

When the AP 910 transmits a sixth beacon frame (step S916), the STA1 921operates by transitioning to the awake state, and acquires the TIMelement included in the beacon frame (step S924). The TIM element is aDTIM that indicates existence of a broadcast frame, and thus the STA1921 receives the broadcast frame transmitted by the AP 910 (step S934)instead of transmitting a PS poll frame to the AP 910.

Meanwhile, the wakeup interval assigned to the STA2 922 may have alonger period than that of the STA1 921. Therefore, the STA2 922 canreceive the TIM element by transitioning to the awake state (step S925)when the fifth beacon frame is transmitted (step S915). The STA2 922knows existence of a frame to be transmitted to the STA2 922 by usingthe TIM element, and transmits a PS poll frame to the AP 910 to requesttransmission (step S925 a). The AP 910 transmits a frame to the STA2 922in response to the PS poll frame (step S933).

In order to operate the PS mode of FIG. 9, the TIM element includes aTIM that indicates whether there is a frame to be transmitted to the STAor a DTIM that indicates whether there is a broadcast/multicast frame.The DTIM may be implemented by configuring a field of the TIM element.

FIG. 10 shows an example of a TIM element format.

Referring to FIG. 10, a TIM element 1000 includes an element ID field1010, a length field 1020, a DTIM count field 1030, a DTIM period field1040, a bitmap control field 1050, and a partial virtual bitmap field1060.

The element ID field 1010 is a field indicating that a correspondinginformation element is a TIM element. The length field 1020 indicates atotal length including this field and its subsequent fields. A maximumvalue may be 255, and its unit may be set to an octet value.

The DTIM count field 1030 informs whether a current TIM element is aDTIM. If it is not the DTIM, the DTIM count field 1030 indicates thenumber of remaining TIM elements until the DTIM is transmitted. The DTIMperiod field 1040 indicates a period according to which the DTIM istransmitted. The DTIM transmission period may be set to a multiple ofthe number of times of transmitting a beacon frame.

The bitmap control field 1050 and the partial virtual bitmap field 1060indicate whether a bufferable frame is buffered for a specific STA. A1^(st) bit of the bitmap control field 1050 indicates whether there is amulticast/broadcast frame to be transmitted. The remaining bits are setto indicate an offset value for interpreting the subsequent partialvirtual bitmap field 1060.

The partial virtual bitmap field 1060 is set to a value that indicateswhether there is a frame to be transmitted to each STA. This may be setin a bitmap format in which a bit value corresponding to an AID value ofthe specific STA is set to 1. According to an AID order, bits can beassigned from 1 to 2007 in sequence. For example, if a 4^(th) bit is setto 1, it implies that traffic to be transmitted to an STA having an AIDof 4 is buffered in an AP.

Meanwhile, when setting a bit sequence of the partial virtual bitmapfield 1060, it may be ineffective to use all bit sequences thatconstitute a bitmap in a situation where there are many consecutive zerobits. For this, offset information for the partial virtual bitmap field1060 may be included in the bitmap control field 1050.

FIG. 11 shows an example of a bitmap control field and a partial virtualbitmap field according to an embodiment of the present invention.

Referring to FIG. 11, a bitmap sequence that constitutes a partialvirtual bitmap field 1060 indicates whether there is buffered frame foran STA having an AID corresponding to a bitmap index thereof. The bitmapsequence constitutes indication information for AIDs from 0 to 2007.

The bitmap sequence can be configured such that 0 is set consecutivelyfrom an initial bit to a k^(th) bit. In addition, the bitmap sequencecan be configured such that 0 is set consecutively from an i^(th) bit toa last bit. This indicates that there is no buffered frame for each ofSTAs assigned with AIDs 0 to k and each of STAs assigned with AIDs i to2007. As such, a size of the TIM element can be decreased in such amanner that offset information is provided for consecutive zerosequences from 0 to k located in a first portion of the bitmap sequenceand by omitting consecutive zero sequences located in a last portionthereof.

For this, a bitmap control field 1050 may include a bitmap offsetsubfield 1051 including offset information of consecutive zero sequencesof the bitmap sequence. The bitmap offset subfield 1051 can be set toindicate k. The partial virtual bitmap field 1060 can be set to includebits from a (k+1)^(th) bit to a (i−1)^(th).

A detailed responding procedure of an STA that receives a trafficindication map (TIM) element may be described with reference to FIGS. 12to 14.

FIG. 12 is a flowchart illustrating an example of a responding procedureof an AP in a TIM protocol.

Referring to FIG. 12, an STA 1220 switches an operation state from adoze state to an awake state in order to receive a beacon frameincluding a TIM from an AP 1210 (S1210). The STA 1220 interprets thereceived TIM element to know a buffered frame to be transmitted theretoexists.

The STA 1220 contends with other STAs in order to access a medium fortransmitting a PS-poll frame (S1220) and transmits the PS-poll frame inorder to request the AP 1210 to transmit a data frame (S1230).

The AP 1210 that receives the PS-poll frame transmitted by the STA 1220transmits the frame to the STA 1220 (S1240). The STA2 1220 receives thedata frame and transmits an acknowledgement (ACK) frame to the AP 1210as a reception response (S1250). Then, the STA2 1220 switches anoperation mode from the awake mode to the doze state (S1260).

As illustrated in FIG. 12, the AP may transmit data at specific timingafter receiving the PS-poll frame unlike an immediate response in whichthe data frame is immediately transmitted when the PS-poll frame isreceived from the STA.

FIG. 13 is a flowchart illustrating another example of a responseprocedure of an AP in a TIM protocol.

Referring to FIG. 13, an STA 1320 switches an operation state from adoze state to an awake state in order to receive a beacon frameincluding a TIM from an AP 1310 (S1310). The STA 1320 interprets thereceived TIM element to know a buffered frame to be transmitted theretoexists.

The STA 1320 contends with other STAs in order to access a medium fortransmitting a PS-poll frame (S1320) and transmits the PS-poll frame inorder to request the AP 1310 to transmit a data frame (S1330).

When the AP 1310 does not prepare the data frame at specific timeinterval like a short interframe space (SIFS) after receiving thePS-poll frame, the data frame is not immediately transmitted but the ACKframe is transmitted to the STA 1320 (S1340), which is thecharacteristic of a deferred response unlike in S1240 where the AP 1210of FIG. 12 immediately transmits the data frame to the STA 1220 tocorrespond to the PS-poll frame.

The AP 1310 performs contention when the data frame is prepared afterthe ACK frame is transmitted (S1350) and transmits the data frame to theSTA 1320 (S1360).

The STA 1320 transmits the ACK frame to the AP 1310 as the receptionresponse to the data frame (S1370) and switches the operation mode fromthe awake state to the doze state (S1380).

When the AP transmits a delivery traffic indication map (DTIM) to theSTA, the procedure of the TIM protocol that proceeds after thetransmission of the DTIM may vary.

FIG. 14 is a flowchart illustrating a procedure of a TIM protocol by aDTIM.

Referring to FIG. 14, STAs 1420 change an operation state from a dozestate to a awake state in order to receive a beacon frame including aTIM element from an AP 1410 (S1410). The STAs 1420 may know that amulticast/broadcast frame is to be transmitted through the receivedDTIM.

The APs 140 transmit the multicast/broadcast frame after transmittingthe beacon frame including the DTIM (S1420). The STAs 1420 switches theoperation state from the awake state to the doze state after receivingthe multicast/broadcast frame transmitted by the AP 1410 (S1430).

In a power save mode operating method based on the TIM protocol that isdescribed with reference to FIGS. 9 to 14, the STAs may confirm whetherbuffered frames to be transmitted exist by traffics buffered through STAidentification information included in the TIM element. The STAidentification information may be information on association identifiers(AID) allotted when the STAs are associated with the AP. The STAidentification information may be configured to directly indicate theAIDs of the STAs related to buffered frames or may be configured as abit map type in which a bit order corresponding to AID values isconfigured by a specific value. The STAs may know that the STAs havebuffered frames when the STA identification information indicates theAIDs of the STAs.

On the other hand, a power management operation based on automatic powersave delivery (APSD) may be provided for the power save of the STA.

The AP that may support the APSD signals that the APSD may be supportedby an APSD subfield in the ability value information fields of a beaconframe, a probe response frame, and a combination response frame. The STAthat may support the APSD uses a power management field in the framecontrol field of a frame in order to indicate whether operation isperformed in an active mode or a power save mode.

The APSD is a mechanism for delivering downlink data and a bufferablemanagement frame to an STA that operates in the power save mode. Theframe transmitted by the STA in the power save mode that is using theAPSD configures the power management bit of the frame control fieldas 1. Therefore, buffering may be caused by the AP.

The APSD defines two delivery mechanisms of unscheduled-APSD (U-APSD)and scheduled-APSD (S-APSD). The STAs may use the U-APSD so that partsor all of the bufferable units (BU) thereof may be delivered in aservice period (SP) that is not scheduled. The STAs may use the S-APSDso that parts or all of the BUs thereof may be delivered.

The STAs that use the U-APSD may not receive the frame transmitted bythe AP in the SP due to interference. The AP may not sense theinterference. However, the AP may determine that the STAs do notcorrectly receive the frame. A U-APSD coexistence ability value has theSTAs indicate requested transmission duration to the AP so that thetransmission duration may be used as an SP for the U-APSD. The AP maytransmit the frame in the SP so that it is possible to improve aprobability of receiving the frame in a state where the STAs areinterfered. In addition, the U-APSD may reduce a probability in whichthe frame transmitted by the AP may not be successfully received in theSP.

The STAs transmit an add traffic stream (ADDTS) request frame includinga U-APSD coexistence element to the AP. The U-APSD coexistence elementmay include information on the requested SP.

The AP may process the requested SP and may transmit an ADDTS responseframe in response to an ADDTS request frame. A state code may beincluded in the ADDTS request frame. The state code may indicateresponse information on the requested SP. The state code may indicatewhether the requested SP is allowed and may further indicate a reasonfor rejection when the requested SP is rejected.

When the requested SP is allowed by the AP, the AP may transmit theframe to the STAs in the SP. The duration of the SP may be specified bythe U-APSD coexistence element included in the ADDTS request frame. Thestart of the SP may be timing at which a trigger frame is transmitted tothe AP so that the AP is normally received.

The STAs may enter the doze state when the U-APSD SP is expired.

In the WLAN system like the HT WLAN system that supports 20 MHz/40 MHzand the VHT wireless LAN system that supports 20 MHz/40 MHz/80MHz/contiguous 160 MHz/non-contiguous 160 MHz (80+80 MHz), servicesthrough multi channel are provided. In the wireless LAN system thatsupports the multi channel, definition on which channel bandwidth is tobe used in each procedure performed for the STA that operates in thepower save mode to receive the buffered frame from the AP is required.

FIG. 15 shows an example of a method for transmitting and receivingframes by an STA that operates in a power save mode according to anembodiment of the present invention.

Referring to FIG. 15, an STA in a doze state enters an awake state inorder to receive a TIM element (S1510).

The STA receives the TIM element (S1520). The TIM element may beincluded in a beacon frame to be transmitted. When the STA receives theTIM element, the STA may determine whether a bufferable frame for theSTA is buffered based on a bitmap sequence of a partial virtual bitmapfield included in the TIM element and the AID of the STA.

The STA that confirms that the bufferable frame is buffered may transmitthe PS-poll frame to request the AP to transmit the buffered frame. Itis necessary to transmit the PS-poll frame through multi channel inorder to receive the buffered frame from the AP through the multichannel in a multi channel WLAN system.

In order to transmit the PS-poll frame through the multi channel, theSTA that operates in the power save mode confirms whether the multichannel to be accessed is in an idle state (S1530). The STA thatperforms backoff in a primary channel confirms CCA for secondarychannels after a backoff timer is expired and determines whether accessto the multichannel may be performed. The PS-poll may be transmittedonly to channels in the idle state that may be accessed.

The STA transmits the PS-poll frame to the AP through the multi channel(S1540). At this time, the PS-poll frame may be transmitted as aduplicate format. That the PS-poll is transmitted as the duplicateformat means that unit PS-poll frames generated for unit bandwidths aretransmitted through a plurality of adjacent channels. Referring to FIG.15, it may be known that the unit PS-poll frames are transmitted throughadjacent CH1 to CH4. When the unit bandwidth is 20 MHz bandwidth, it maymean that the 20 MHz PS-poll frames are transmitted through the CH1 tothe CH4. For transmitting the PS-poll frame by the duplicate format, theAP may replicate a transmission of a PS-poll frame on a primary channelin at least one secondary channel.

Each of the unit bandwidth PS-poll frames of the PS-poll framestransmitted by the duplicate format may be realized by an individualPPDU format. That is, the unit PS-poll frames may have the formatsillustrated in FIG. 4 for a single receiver.

The PS-poll frame may include information on transmission bandwidth. TheSTA may configure a CH_BANDWIDTH_IN_NON_HT parameter of TXVECTOR that isa transmission parameter as a value of bandwidth in which the entireunit PS-poll frames are transmitted in generating the PS-poll frame.Information on the CH_BANDWIDTH_IN_NON_HT parameter may be included in ascrambling sequence used for processing data fields included in the unitPS-poll frames, which will be described in detail.

Initial 7 bits in the scrambling sequence used for scrambling the datafield including a PSDU, a service field, and a tail field may becommonly realized by a 7 bit pseudorandom nonzero integer sequence. Whena specific PPDU is transmitted in accordance with the duplicate format,information indicating the bandwidth in which the plurality of unitPPDUs (the PPDUs of the duplicate format) are transmitted may beincluded in the scrambling sequence for scrambling the data fields ofthe unit PPDUs transmitted through the channels.

In addition, it is necessary that additional information that mayindicate that bandwidth information is included in the scramblingsequence be included. For this purpose, an address field, e.g. atransmitter address (TA) field, in the MAC headers of each unit PS-pollframe may be configured as a bandwidth signaling TA. The bandwidthsignaling TA indicates a MAC address of the STA transmitting thecorresponding frame, and a individual/group bit in the MAC address maybe set to ‘1’ The bandwidth signaling TA may indicate that additionalsignaling information on the bandwidth signaling TAs is included. Indetail, the bandwidth signaling TA may indicate that bandwidthinformation on the CH_BANDWIDTH_IN_NON_HT parameter is included in thescrambling sequence.

On the other hand, when a PPDU is commonly transmitted, bandwidthinformation may be included in a VHT-SIG-A field of the PPDU. Ingenerating the PPDU, the transmitting STA may set the CH_BANDWIDTHparameter of a transmission parameter TXVECTOR to indicate bandwidthinformation for transmitting the PPDU. In realizing the PPDU based onthe transmission parameter, the STA may configure bandwidth informationin the BW field of the VHT-SIG-A field based on the value configured inthe CH_BANDWIDTH parameter.

When the transmitting STA configures the TA field as a bandwidthsignaling TA to generate the PPDU, CH_BANDWIDTH may be configured to bethe same as CH_BANDWIDTH_IN_NON_HT.

That is, first bandwidth information indicating the entire bandwidth inwhich the PS-poll frame is transmitted may be included in the signalfields (the VHT-SIG-A fields) of the unit PS-poll frames which areincluded in the PS-poll frame and transmitted over the channels. Inaddition, the data fields may be scrambled based on the scramblingsequence including second bandwidth information indicating the entirebandwidth.

Referring to FIG. 15, the AP that receives the PS-poll frames from theSTAs transmits the buffered frame to the STA in response to the PS-pollframe (S1550). In transmitting the buffered frame to the STA, the AP maytransmit the buffered frames through bandwidth equal to or smaller thanthe bandwidth in the PS-poll frame is transmitted. For example, the APmay receive at least one unit PS-poll frame among the PS-poll frame ofthe duplicate format. In this case, the AP may transmit the bufferedframe only through the channels that normally receive the at least oneunit PS-poll frame. For another example, although the AP receives the atleast one unit PS-poll frame among the PS-poll frames of the duplicateformat, the buffered frame may be transmitted using the entire bandwidthin which the PS-poll frame of the duplicate format is transmitted. Onthe other hand, the channel determined not to be in the idle statethrough the CCA immediately before the AP receives the PS-poll frame maynot be used for transmitting the buffered frame.

The STA receive the buffered frame from the AP and transmits an ACKframe to the AP (S1560). The ACK frame may be transmitted only through aprimary channel. The ACK frame may be transmitted through the channelswhere the PS-poll frame is transmitted. The ACK frame may be transmittedthrough the channels where the buffered frame is transmitted.

The STAs may enter the doze state after transmitting the ACK frame(S1570).

According to the above-described frame transmitting and receivingmethod, in the multi channel WLAN system, the STA may transmit thePS-poll frame of the duplicate format to request to transmit thebuffered frame and may signal bandwidth for transmitting the bufferedframe. Therefore, the AP may transmit the buffered frame to the STAbased on information on the bandwidth signaled by the PS-poll frame.

FIG. 16 shows another example of a method for transmitting and receivingframes by an STA operating in a power save mode according to anembodiment of the present invention. FIG. 16 shows a method fortransmitting and receiving frames based on deferred response.

Referring to FIG. 16, a STA in a doze state enters an awake state inorder to receive a TIM element (S1610).

The STA receives the TIM element (S1620). The TIM element may beincluded in a beacon frame to be transmitted. When the STA receives theTIM element, the STA may determine whether a bufferabe frame therefor isbuffered based on a bitmap sequence of a partial virtual bitmap fieldincluded in the TIM element and the AID of the STA.

The STA that confirms that the bufferabe frame is buffered may transmita PS-poll frame to request the AP to transmit the buffered frame. In amulti channel WLAN system, in order to receive the buffered frame fromthe AP through the multi channel, it is necessary to transmit thePS-poll frame through the multichannel.

In order to transmit the PS-poll frame through the multi channel, theSTA that operates in the power save mode confirms whether access to themulti channel to be accessed may be performed (S1630). The STA thatperforms backoff by a primary channel confirms CCA for secondarychannels after a backoff timer is expired and determines whether accessto the multichannel may be performed. The PS-poll frame may betransmitted only to the channels in an idle state that may be accessed.

The STA transmits the PS-poll frame to the AP through the multi channel(S1640). At this time, the PS-poll frame may be transmitted by aduplicate format as illustrated in S1550 of FIG. 15. A detailed methodof transmitting the PS-poll frame and configuration of the PS-poll framemay follow the method and configuration described in detail in S1550 ofFIG. 15.

The AP that receives the PS-poll frame from the STA may transmit thebuffered frame as a response to the PS-poll frame. After receiving thePS-poll frame, the buffered frame to be transmitted may not be prepared.In this case the AP transmits an ACK frame as a response to the PS-pollframe like in the deferred response of FIG. 13 (S1650). The ACK framemay be transmitted on a primary channel. The ACK frame may betransmitted on a plurality of channels. In case where the AC frame istransmitted on the plurality of channels by duplicate format, theplurality of channels corresponds to the plurality of channels on whichthe PS-poll frame is transmitted. Alternatively, the plurality ofchannels corresponds to at least one channel over which the AP hasnormally received at least one unit PS-poll frame transmitted in thePS-poll frame of duplicate format

On the other hand, the value of a receiver address (RA) field includedin the ACK frame transmitted for the AP may be configured as the MACaddress of the STA that receives the ACK frame. In detail, the RA fieldof the ACK frame may be configured from the TA field of the PS-pollframe. In the example, the TA field of the PS-poll frame is configuredas a bandwidth signaling TA. In this case, the RA field of the ACK framemay be obtained from the TA field of the PS-poll frame, which may beconfigured as a non-bandwidth signaling TA. The non-bandwidth signalingTA may be obtained by configuring the individual/group bit of thebandwidth signaling TA as ‘0’.

After the ACK frame is transmitted, when a buffered frame to betransmitted is prepared, the AP confirms whether access to themultichannel may be performed in order to transmit the buffered framethrough the multichannel (S1660). The STA that performs backoff by aprimary channel confirms CCA for secondary channels after a backofftimer is expired and determines whether access to the multichannel maybe performed. The buffered frame may be transmitted only to the channelsin an idle state that may be accessed.

The AP transmits the buffered frame to the STA (s1670). In transmittingthe buffered frame to the STA, the AP may transmit the buffered framethrough bandwidth equal to or smaller than the bandwidth in which thePS-poll frame is transmitted. For example, the AP may receive at leastone unit PS-poll frame among the PS-poll frame of the duplicate format.In this case, the AP may transmit the buffered frame only throughchannels that normally receive the at least one PS-poll frame. Foranother example, although the AP receives the at least one unit PS-pollframe among the PS-poll frame of the duplicate format, the bufferedframe may be transmitted using the entire bandwidth in which the PS-pollframe of the duplicate format is transmitted. On the other hand, thechannels determined not to be in the idle state through CCA immediatelybefore the AP receives the PS-poll frame may not be used fortransmitting the buffered frame.

The STA receives the buffered frame from the AP and transmits the ACKframe to the AP (S1680). The ACK frame may be transmitted only through aprimary channel. The ACK frame may be transmitted through the channelswhere the PS-poll frame is transmitted. The ACK frame may be transmittedthrough the channels where the buffered frame is transmitted.

The STA may enter a doze state after transmitting the ACK frame (S1690).

In the embodiment of the present invention described in detail withreference to FIGS. 15 and 16, the AP transmits the PS-poll frame of theduplicate format to the STA through four adjacent channels and receivesthe buffered frame from the AP through no more than four channels as aresponse to the PS-poll frames. The number and proximity of channels fortransmitting and receiving a frame are not limited. For example, the APand/or the STA may transmit/receive the PS-poll frame and the bufferedframe through two non-adjacent channel groups including adjacentchannels. For example, in a next generation wireless LAN system,transmission and reception using non-contiguous 160 MHz bandwidth (80+80MHz) may be applied to the frame transmitting and receiving methodaccording to the present invention. When the unit bandwidth of thechannel is 20 MHz, transmission/reception of the duplicated frame may beperformed as follows.

1) 40 MHz duplicate format: transmission of a frame through a 20 MHzchannel is duplicated so that the duplicated frame is transmittedthrough two adjacent 20 MHz channels.

2) 80 MHz duplicate format: transmission of a frame through a 20 MHzchannel is duplicated so that the duplicated frame is transmittedthrough four adjacent 20 MHz channels.

3) Contiguous 160 MHz duplicate format: transmission of a frame througha 20 MHz channel is duplicated so that the duplicated frame istransmitted through eight adjacent 20 MHz channels.

4) Non-contiguous 160 MHz (80+80 MHz) duplicate format: transmission ofa frame through a 20 MHz channel is duplicated so that the duplicatedframe is transmitted through two channel groups each including fouradjacent channels and that the two channel groups are not adjacent toeach other.

On the other hand, according to the frame transmitting and receivingmethod based on the conventional TIM protocol, the STAs may know whetherbufferable frame therefor is buffered through the TIM elementtransmitted by the AP. In this case, the STAs transmit the PS-poll frameto the AP based on a PS-poll mechanism to request to transmit thebuffered frame. The AP receives the PS-poll frame and accesses channelsthrough contention to transmit the frame to the STA. In this case, theAP may transmit one frame (PSDU) to the STA at one time. Therefore, whenthe amount of the buffered traffic for a specific STA is large, it isinefficient to process traffic.

In addition, exchange of RTS/CTS frames required when data istransmitted in order to prevent a hidden node problem causes a largeamount of overhead to data transmission. In addition, in the U-APSD, ittakes long for the STAs to transmit a trigger frame and to request theAP to transmit data and for the AP to prepare data to be transmitted tothe STAs and to perform contention for data transmission. Since the STAsmay unnecessarily maintain the awake state for the corresponding time,the efficiency of the power save may be deteriorated.

In order to provide the efficient frame transmitting and receivingmethod for the STA that operates in the power save mode, the U-APSD maybe applied to the TIM protocol. The STA may receive a frame no less thanonce from the AP through the SP therefor. For this purpose, the STA mayrecognize that the buffered frame to be transmitted thereto by the APexists through the TIM element of the beacon frame. Then, the STA maytransmit the trigger frame to the AP to announce that the SP thereof isstarted and may request the AP to transmit the buffered frame for thebuffered traffic.

For this purpose, a SP-poll frame is suggested by the present invention.

FIG. 17 is a block diagram illustrating an MAC frame format of anSP-poll frame according to an embodiment of the present invention.

Referring to FIG. 17, a SP-poll frame 1700 may include a frame controlfield 1710, a duration field 1720, a BSSID (RA) field 1730, a TA field1740, a frame body 1750, and an FCS field 1760.

The frame control field 1710 may indicate that the frame is an SP-pollframe.

The duration field 1720 may indicate the duration of the polled SPinitiated by the SP-poll frame 1700. The duration field 1720 may be abasis of configuring the network allocation vector (NAV) of another STAthat does not transmit the SP-poll frame 1700.

The BSSID (RA) field 1730 may include identification information on aBSS operated by the AP with which the STA is combined or identificationinformation on the AP. The identification information may be BSSID.

The TA field 1740 may include identification information on the STA thattransmits the SP-poll frame 1740. The identification information may bethe MAC address of the STA. The identification information may includethe AID of the STA.

The frame body 1750 may include a polled SP interval field indicatinginterval from timing at which the SP initiated by the SP-poll frame 1700is terminated and a next SP is initiated.

The FCS field 1760 may include a sequence for CRC.

The STA may transmit the SP-poll frame to the AP to announce the SP ofthe STA to the AP. The SP initiated by the SP-poll frame may be referredto as a polled SP. The STA may request the AP to transmit data using theSP-poll frame. The AP may transmit the buffered frame to the STA in theinitiated polled SP.

A frame transmitting and receiving method by the power save mode STAbased on the SP-poll may be divided into an immediate SP-poll mechanismand a deferred SP-poll mechanism in accordance with the response of theAP that receives the SP-poll frame.

FIG. 18 shows an example of a method for transmitting and receivingframes by an STA that operates in a power save mode according to anotherembodiment of the present invention. The method for transmitting andreceiving frames of FIG. 18 may be an example of a method transmittingand receiving frames according to an immediate SP-poll mechanism.

Referring to FIG. 18, the STA in the doze state enters the awake statein order to receive a TIM element (S1810).

The STA receives the TIM element (S1820). The TIM element may beincluded in a beacon frame to be transmitted. The TIM element may beincluded in a beacon frame to be transmitted. When the STA receives theTIM element, the STA may determine whether a bufferable frame thereforis buffered based on the bitmap sequence of a partial virtual bitmapfield included in the TIM element and the AID of the STA.

The STA that confirms that the bufferable frame is buffered may obtain achannel access authority through contention and may transmit the SP-pollframe to request the AP to transmit the buffered frame (S1830).

The AP that receives the SP-poll frame transmits the at least onebuffered frame to the STA after SIFS (S1841, S1842, and S1843). In thiscase, the AP may continuously transmit a plurality of buffered frames inthe polled SP.

When a specific polled SP is not configured through additional signalingbetween the AP and the STA, an end of service period (EOSP) value may beconfigured as 1 in the final buffered frame transmitted by the AP to theSTA in the polled SP. Therefore, the polled SP between the STA and theAP may be terminated.

On the other hand, a specific polled SP may be configured throughadditional signaling between the AP and the STA. For this purpose, theduration field of the SP-poll frame transmitted by the STA may beapplied. In this case, the polled SP may be initiated at the timing whenthe STA transmits the SP-poll frame or when the AP receives the SP-pollframe. The polled SP may be configured from the initiation timing forthe duration indicated by the duration field. The AP may transmit thebuffered frame in accordance with polled SP duration. The STA mayreceive the buffered frame in accordance with the polled SP duration.

The STA may transmit the ACK frame to the AP when the polled SP isterminated (S1850). The STA enters the doze state after transmitting theACK frame (S1860).

FIG. 19 shows another example of a method for transmitting and receivingframes by an STA that operates in a power save mode according to anotherembodiment of the present invention. The method for transmitting andreceiving frames of FIG. 19 is based on the deferred SP-poll mechanism.

Referring to FIG. 19, an STA in a doze state enters an awake state inorder to receive a TIM element (S1910).

The STA receives the TIM element (S1920). The TIM element may beincluded in a beacon frame to be transmitted. The TIM element may beincluded in a beacon frame to be transmitted. When the STA receives theTIM element, the STA may determine whether a bufferable frame thereforis buffered based on the bitmap sequence of a partial virtual bitmapfield included in the TIM element and the AID of the STA.

The STA that confirms that the bufferable frame is buffered may obtain achannel access authority through contention and may transmit the SP-pollframe to request the AP to transmit the buffered frame (S1930). A firstpolled SP may be initiated by transmission of the SP-poll frame.

On the other hand, the AP receives the SP-poll frame and may nottransmit the buffered frame to the STA in SIFS. In this case, the APtransmits an ACK frame to the STA after receiving the SP-poll frame(S1940).

The STA that receives the ACK frame as a response to the transmittedSP-poll frame may recognize that the AP may not transmit the bufferedframe. In this case, the first polled SP initiated by the transmissionof the SP-poll frame may be terminated. The STA receives the ACK frameand enters the doze state (S1950).

On the other hand, the STA enters an awake state at the timing indicatedby the polled SP interval field of the SP-poll frame (S1960) andtransmits the SP-poll frame to the AP (S1970).

On the other hand, the AP may previously know the timing at which asecond polled SP starts through the polled SP interval field of theSP-poll frame received in S1930. Therefore, the AP may receive theSP-poll frame and may previously prepare the buffered frame to betransmitted to the STA after the SIFS so that the AP may transmit the atleast one buffered frame to the STA (S1981, S1982, S1983, and S1984).

The duration of the second polled SP initiated by the STA transmittingthe SP-poll frame (S1970) may be specified by the duration of the polledperiod described with reference to FIG. 18. That is, the polled durationmay be terminated by transmitting the buffered frame including the EOSPfield in which the AP is configured as ‘1’. That is, the polled durationmay be terminated by the AP transmitting the buffered frame includingthe EOSP field configured as ‘1’. The second polled duration may bespecified by the duration indicated by the duration field of the SP-pollframe transmitted by the STA in S1970.

The STA transmits the ACK frame to the AP (S1990) when the second polledSP is terminated and may enter the doze state (S1995).

When the STA obtains the buffered frame from the AP based on the frametransmitting and receiving method according to the above-describedembodiment, a device for preventing collision with frames transmittedand received by other STAs may be necessary. For this purpose, the otherSTAs may configure NAVs based on the SP-poll frame transmitted by theSTA.

FIG. 20 shows still another embodiment of a method for transmitting andreceiving frames according to the embodiment of the present invention.In FIG. 20, it is assumed that an STA1 and an STA3 are positioned in theservice coverage of the AP and that the STA2 is positioned in thecoverage of the STA1.

Referring to FIG. 20, the STA1 enters an awake state in order to receivea TIM element (S2010) and receives the TIM element (S2020).

The STA that confirms that the bufferable frame is buffered based on theTIM element transmits the SP-poll frame to the AP (S2030).

The AP may transmit the buffered frame to the STA in the polled SPinitiated by the transmission of the SP-poll frame (S2041 and S2042).

When the polled SP is terminated, the STA1 transmits an ACK frame to theAP (S2050) and enters a doze state (S2060).

Since the STA2 is positioned outside the service coverage of the AP, theSTA2 may not receive the frame transmitted by the AP. On the other hand,since the STA2 is positioned in the coverage of the STA1, the STA2 mayreceive the frame transmitted by the STA1. The STA2 may overhear theSP-poll frame transmitted by the STA1 (S2071). Therefore, the STA2 mayconfirm the duration of the polled SP through the duration field of theSP-poll frame and may set an NAV for the duration (S2072). The NAV isset by the STA2 so that collision between the STA1 and the STA2 may beprevented.

Since the STA3 is positioned in the service coverage of the AP, the STA3may receive the frame transmitted by the AP. On the other hand, the STA3is positioned outside the coverage of the STA1, the STA3 may not receivethe frame transmitted by the STA1. In this case, the STA3 may overhearthe buffered frame transmitted by the AP (S2081 and S2082).

The STA3 may set an NAV based on duration information included in thepreamble and/or the MAC header of the transmitted buffered frame (S2091and S2092). Therefore, collision between the STA3 and the AP may beprevented.

When the SP-poll frame is applied to the frame transmission andreception of the STA that operates in the power save mode, the bufferedframe may be transmitted at least one time in the polled SP. Therefore,traffic may be efficiently processed.

The above-described SP-poll frame may be applied to the frametransmitting and receiving method in the multi channel system.

FIG. 21 shows a method for transmitting and receiving frames by an STAthat operates in a power save mode according to still another embodimentof the present invention.

Referring to FIG. 21, an STA in a doze state enters an awake state inorder to receive a TIM element (S2110).

The STA receives the TIM element (S2120). The TIM element may beincluded in a beacon frame to be transmitted. The TIM element may beincluded in a beacon frame to be transmitted. When the STA receives theTIM element, the STA may determine whether a bufferable frame thereforis buffered based on the bitmap sequence of a partial virtual bitmapfield included in the TIM element and the AID of the STA.

The STA that confirms that the bufferable frame is buffered may transmitthe SP-poll frame to request the AP to transmit the buffered frame. Thepolled SP may be initiated by the STA transmitting the SP-poll frame. onthe other hand, according to the embodiment, since frame transmissionand reception between the AP and the STA is performed by the multichannel WLAN system, in order for the STA to receive the buffered framefrom the AP through the multi channel, it is necessary to transmit theSP-poll frame through the multi channel.

In order to transmit the SP poll frame through the multichannel, the STAthat operates in the power save mode confirms whether the multi channelto be accessed is in an idle state (S2130). The STA that performsbackoff in a primary channel confirms CCA for secondary channels after abackoff timer is expired and determines whether access to the multichannel may be performed. The SP-poll may be transmitted only tochannels in the idle state that may be accessed.

The STA transmits the SP-poll frame to the AP through the multi channel(S2140). At this time, the SP-poll frame may be transmitted by aduplicate format. That the SP-poll is transmitted by the duplicateformat means that unit SP-poll frames generated for unit bandwidths aretransmitted through a plurality of adjacent channels. Referring to FIG.21, it may be known that the unit SP-poll frames are transmitted throughadjacent to CH4. When the unit bandwidth is 20 MHz bandwidth, it maymean that the 20 MHz SP-poll frames are transmitted through the CH1 tothe CH4. The transmission of the SP-poll frame of the duplicate formatis not limited to transmission through four adjacent channels. Adetailed channel use example may follow the channel use example fortransmitting the duplicate format described with reference to FIGS. 15and 16.

Each of the unit SP-poll frames of the SP-poll frame transmitted by theduplicate format may be realized by an individual PPDU format. That is,the unit SP-poll frames may have the formats illustrated in FIG. 4 for asingle receiver.

The SP-poll frame may include information on transmission bandwidth. TheSTA may configure a CH_BANDWIDTH_IN_NON_HT parameter of TXVECTOR that isa transmission parameter as a value of bandwidth in which the entireunit SP-poll frames are transmitted in generating the SP-poll frame.Information on the CH_BANDWIDTH_IN_NON_HT parameter may be included in ascrambling sequence used for processing data fields included in the unitSP-poll frames. The information may be included in initial 7 bits in thescrambling sequence. In addition, an address field including the MACheaders of each unit SP-poll frame may be configured by theabove-described bandwidth signaling TA.

The value of the CH_BANDWIDTH parameter of the TXVECTOR parameter usedwhen the STA that desires to transmit the SP-poll frame generates theSP-poll frame may be configured in the BW field of the VHT-SIG-A fieldof the unit SP-poll frame. When the TA field of the unit SP-poll isconfigured as the bandwidth signaling TA, the value of the CH_BANDWIDTHparameter may be configured to the as the value ofCH_BANDWIDTH_IN_NON_HT.

That is, first bandwidth information indicating the entire bandwidth inwhich the SP-poll frame is transmitted may be included in the signalfields (the VHT-SIG-A fields) of the unit SP-poll frames included in theSP-poll frame transmitted by the STA and transmitted over the channels.In addition, the data fields of the unit SP-poll frames may be scrambledbased on the scrambling sequence including second bandwidth informationindicating the entire bandwidth.

Referring back to FIG. 21, the AP that receives the SP-poll frame fromthe STAs transmits the buffered frames to the STA for the polled SPinitiated by the SP-polled frame (S2151, S2152, and S2153). Intransmitting the buffered frames to the STAs, the AP may transmit thebuffered frames through bandwidth equal to or smaller than the bandwidthin the SP-poll frame is transmitted. For example, the AP may receive atleast one unit SP-poll frame among the SP-poll frame of the duplicateformat. In this case, the AP may transmit the buffered frames onlythrough the channels that normally receive the at least one unit SP-pollframe. For another example, although the AP receives at least one unitSP-poll frames among the SP-poll frame of the duplicate format, thebuffered frames may be transmitted using the entire bandwidth in whichthe SP-poll frame of the duplicate format are transmitted. On the otherhand, the channel determined not to be in the idle state through the CCAimmediately before the AP receives the SP-poll frame may not be used fortransmitting the buffered frame.

In transmitting the buffered frames a plurality of number of times inthe polled SP, transmission bandwidth for a later transmitted bufferedframe may be equal to or smaller than transmission bandwidth for apreviously transmitted buffered frame.

The STAs receive the buffered frames from the AP and transmits an ACKframe to the AP (S2160). The ACK frame may be transmitted only through aprimary channel. The ACK frame may be transmitted through the channelswhere the SP-poll frame is transmitted. The ACK frame may be transmittedthrough the channels where the buffered frame is transmitted.

The STAs may enter the doze state after transmitting the ACK frame(S2170).

According to the above-described frame transmitting and receivingmethod, in the multi channel WLAN system, the STAs may transmit theSP-poll frame of the duplicate format to request to transmit thebuffered frames and the polled SP for the transmission of the bufferedframes no less than one time may be initiated. In addition, thebandwidth in which the AP transmits the buffered frames may be signaledby the transmission of the duplicate format. Therefore, the AP maytransmit the at least one buffered frame to the STA based on thebandwidth signaled in the polled SP initiated by the SP-poll frame.

FIG. 22 is a block diagram illustrating a wireless device by which anembodiment of the present invention may be realized.

Referring to FIG. 22, a wireless device 2200 includes a processor 2210,a memory 2220, and a transceiver 2230. The transceiver 2230 transmitsand/or receives a wireless signal and realizes a physical layer of IEEE802.11. The processor 2210 may be configured to be operably connected tothe transceiver 2230 to transmit and receive a TIM element and todetermine whether the bufferable frame therefor is buffered. Theprocessor 2210 may be configured to transmit the PS-poll frame of theduplicate format or the SP poll frame. The processor 2210 may beconfigured to transmit and receive the buffered frame. The processor2110 may be configured to change a doze state and/or an awake state inaccordance with the transmission and reception of the TIM element andthe buffered frame. The processor 2210 may be configured to realize theabove-described embodiment of the present invention with reference toFIGS. 15 to 21.

The processor 2210 and/or the transceiver 2230 may include anapplication-specific integrated circuit (ASIC), different chip sets, alogic circuit, and/or a data processing apparatus. When the embodimentis realized by software, the above-described method may be realized by amodule (a procedure, a function, etc.) for performing theabove-described function. The module is stored in the memory 2220 andmay be executed by the processor 2210. The memory 2220 may be includedin the processor 2210 and may be positioned outside to be functionallyconnected to the processor 2210 by various well-known means.

In the above-described exemplary system, the methods are described basedon the flowcharts as a series of steps or blocks. However, the presentinvention is not limited to the order of the steps. A certain step maybe generated in a different order from the above-described other stepsor may be simultaneously generated with the other steps. In addition,those who skilled in the art may understand that the steps in theflowcharts are not exclusive but other steps may be included or at leastone step may be deleted without affecting the scope of the presentinvention.

The invention claimed is:
 1. A method for managing a power in a wirelesslocal area network, the method comprising: transmitting, by a station(STA) in a power save mode, a PS poll frame to an access point (AP) in afirst 20 MHz channel; transmitting, by the STA in the power save mode,at least one duplicated PS poll frame to the AP in at least one second20 MHz channel, the at least one duplicated PS poll frame beinggenerated by duplicating the PS poll frame; and receiving, by the STA, abufferable frame from the AP as a response of at least one of the PSpoll frame and the at least one duplicated PS poll frame.
 2. The methodof claim 1, wherein the PS poll frame and the at least one duplicated PSpoll frame respectively comprise a data field, wherein the data field isscrambled based on a specific scrambling sequence, wherein the datafield comprises a transmitter address (TA) field set to a bandwidthsignaling TA, and wherein the bandwidth signaling TA indicates anaddress of the STA, and the specific scrambling sequence includes firstbandwidth information.
 3. The method of claim 2, wherein the firstbandwidth information indicates a bandwidth for the PS poll frame andthe at least one duplicated PS poll frame.
 4. The method of claim 3,wherein the PS poll frame and the at least one duplicated PS poll framefurther respectively comprises a VHT-SIG-A field including a bandwidthfield, wherein the VHT-SIG-A field is generated based on secondbandwidth information, and wherein the second bandwidth information issame as the first bandwidth information.
 5. The method of claim 2,further comprising: receiving, by the STA in the power save mode, anacknowledgement (ACK) frame from the AP for acknowledging the PS pollframe and the at least one duplicated PS poll frame.
 6. The method ofclaim 5, wherein the ACK frame comprises a receiver address (RA) field,wherein the RA field is set to a non-bandwidth signaling TA obtained inbasis of the TA field in at least one of the PS poll frame and the atleast one duplicated PS poll frame.
 7. The method of claim 1, whereinthe number of the at least one secondary 20 MHz channel is one, andwherein the first 20 Mhz channel is contiguous with the secondary 20 MHzchannel.
 8. The method of claim 1, wherein the number of the at leastone secondary 20| MHz channel is three, and wherein the first 20 MHzchannel is contiguous with the contiguous three secondary 20 MHzchannels.
 9. The method of claim 1, wherein the number of the at leastone secondary 20 MHz channel is seven, and wherein the first 20 MHzchannel is contiguous with the contiguous seven secondary 20 MHzchannels.
 10. The method of claim 1, wherein the number of the at leastone secondary 20 MHz is seven, wherein the first 20 MHz channel iscontiguous with three 20 MHz secondary channels among the sevensecondary 20 MHz channels, and wherein the first 20 MHz channel and thethree 20 MHz secondary channels are not contiguous with the remainingfour secondary 20 MHz channels.
 11. The method of claim 1, furthercomprising receiving, by the STA in the power save mode, a TrafficIndication Map (TIM) element from the AP; and determining, by the STA inthe power save mode, whether the bufferable frame is buffered for theSTA based on the TIM element; wherein if it is determined that thebufferable frame is buffered, the PS poll frame and the at least one PSpoll frame is transmitted.
 12. A wireless apparatus in a wireless localarea network comprising: a transceiver transmits and receives frames;and a processor is configures to the transceiver, wherein the processoris configured to: transmit a power save (PS) poll frame to an accesspoint (AP) in a first 20 MHz channel, transmit at least one duplicatedPS poll frame to the AP in at least one second 20 MHz channel, the atleast one duplicated PS poll frame being generated by duplicating the PSpoll frame, and receive a bufferable frame from the AP as a response ofat least one of the PS poll frame and the at least one duplicated PSpoll frame.